package com.zyf.swordRefersOffer;

import java.text.DecimalFormat;
import java.util.Scanner;

public class _08_ {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		System.out.println(sum(n));
	}

	public static double sum(int n) {
		double sum = 0.0;
		for (int i = 1; i <= n; i++) {
			sum += 1.0 / (5 * (2 * i - 1)) - 1.0 / (5 * 2 * i);
		}
		DecimalFormat df = new DecimalFormat("#.0000");
		return Double.parseDouble(df.format(sum));
	}
}
